A joint generally consists of two relatively rigid bony structures that maintain a relationship with each other. Soft tissue structures spanning the bony structures hold the bony structures together and aid in defining the motion of one bony structure to the other. In the knee, for example, the bony structures are the tibia and the femur. Soft tissue such as ligaments, tendons, menisci, and capsule provide support to the tibia and femur. A smooth and resilient surface consisting of articular cartilage covers the bony structures. The articular surfaces of the bony structures work in concert with the soft tissue structures to form a mechanism that defines the envelop of motion between the structures. Within a typical envelop of motion, the bony structures move in a predetermined pattern with respect to one another. When fully articulated, the motion defines a total envelop of motion between the bony structures. The soft tissue structures spanning the knee joint tend to stabilize the knee in a transverse plane. This transverse stability enables the bony structures to slide and rotate on one another in an orderly fashion.
The articular surfaces are subject to a variety of diseases, accidents and the like that cause the surfaces to be damaged. A common disorder of joints is degenerative arthritis. Degenerative arthritis causes progressive pain, swelling, and stiffness of the joints. As the arthritic process develops, the joint surfaces wear away, resulting in contractures of the surrounding soft tissues that provide stability to the joint. Changes in the articular surfaces resulting from arthritis decrease stability and increase the translation of the joint.
Treatment of the afflicted articular bone surfaces depends, among other things, upon the severity of the damage to the articular surface and the age and general physical robustness of the patient. The end result commonly necessitates joint replacement surgery wherein the articulating elements of the joint are replaced with artificial elements commonly consisting of a part made of metal articulating with a part made of ultra high molecular weight polyethylene (UHMWPE).
A relatively young patient with moderate to severe degeneration of the knee joint is often treated with drug therapies. While drug therapies may temporarily provide relief of pain, progression of the disease, with resulting deformity and reduced function, ultimately necessitates surgery. Alternative treatments such as nonsteroidal anti-inflammatory drugs, cortisone injections, and arthroscopic debridement similarly provide only temporary relief of symptoms.
In severe situations, the entire articular surface of a bone may be replaced with an artificial surface, as, for example, when condyles at the distal end of the femur are largely replaced with a prosthetic device having polished metal condyles and the tibial plateau is replaced with a plastic bearing that may be supported by a metal component. Joint replacement surgery has become a proven and efficacious method of alleviating pain and restoring function of the joint.
Current methods of preparing the intraarticular rigid elements of a joint to receive components as in joint replacement surgery involve an extensive surgical exposure. The exposure must be sufficient to permit the introduction of guides that are placed on, in, or attach to the joint, along with cutting blocks to guide the use of saws, burrs and other milling devices, and other instruments for cutting or removing cartilage and bone that subsequently is replaced with artificial surfaces. The distal end of the femur may be sculpted to have flat anterior and posterior surfaces generally parallel to the length of the femur, a flat end surface normal to the anterior and posterior surfaces, and angled flat surfaces joining the above mentioned surfaces, all for the purpose of receiving a prosthetic device.
A full joint replacement, using the example of the knee joint, also requires the proximal end of the tibia to be sculpted to receive a prosthesis having a generally upwardly facing bearing surface mimicking the normal tibial bearing surface and designed to articulate with the condylar surfaces of the femoral prosthesis. Typically, this surgery is performed with instruments or guides to orient cutting blocks, such that the preparation of the bone is in concordance with the correct alignment of the limb and the parts are correctly oriented in both coronal and sagittal positions. The guides are placed on exposed bones and generally reference anatomical points on that bone to establish a resection plane. For instance, with total knee replacement, arthroplasty guides are used by referencing, for example, the intramedullary cavity and the epicondylar and posterior condylar axes.
Knee joint prosthesis of the type referred to above are well known, and are described, for example, in Caspari et. al., U.S. Pat. Nos. 5,171,244, 5,171,276 and 5,336,266, Brown, U.S. Pat. No. 4,892,547, Burstein et al., U.S. Pat. No. 4,298,992, and Insall et. al., U.S. Pat. 6,068,658.
Substantial effort has been made to provide appropriate degrees of curvature to the condyles. For example, the earlier mentioned U.S. Pat. Nos. 5,171,276, 4,298,992 and 6,068,658 show that the radius of curvature in the anterior-posterior direction of the condyle of a femoral prosthesis may be somewhat greater near the anterior portion of the condyle than near the posterior portion. Kester et al., U.S. Pat. No. 5,824,100 teaches that a portion of this curvature of the condyle may be formed about a constant radius having its origin along a line between the lateral and medial collateral ligament attachment points on the femur.
Historically, a variety of modular prosthetic joint implants have been developed. The following descriptions of modular implants relate specifically to the knee. Early designs for knee implants, called polycentric knee implants, were developed with separate components for the medial and lateral compartments. Additionally, modular fixed-bearing knee implants having a polyethylene insert that is held relatively rigidly in place have been developed. Alternately, there are mobile bearing knee implants wherein the polyethylene bearing is designed to slide or move with minimal or no constraint on a tibial baseplate. Furthermore, both meniscal bearing and fixed bearing knee implants have been developed including either separate polyethylene bearings or a single polyethylene bearing that resides on a metallic tibial baseplate. While implant systems have been developed with fixed bearing elements or mobile bearing elements on the medial and lateral sides of the tibiofemoral joint, systems have not been developed having a combination of a fixed bearing on one side and a mobile bearing on the other side of the tibiofemoral joint.
Mobile bearing tibial implants may be configured to be more congruent with the femoral side of a knee arthroplasty, yielding lower contact stress. The resultant lower contact stress reduces the. possibility of damage sometimes encountered with some fixed bearing designs wherein the yield strength of the bearing material is exceeded. In general, fixed bearing implant designs are. less difficult to properly align and balance than mobile bearing designs. Mobile bearing designs are frequently desirable to reduce contact stress and the resulting wear of the bearing surface. However, with mobile bearing designs, there is the possibility of the bearing becoming dislodged from the implant. Additionally, mobile bearing knee designs are more surgically demanding to implant then fixed bearing designs.
The combination of a fixed bearing insert for the medial compartment and a mobile bearing insert for the lateral compartment is particularly attractive because the lateral femoral condyle rolls backward on the lateral tibial plateau as much as 10 to 20 mm whereas the medial condyle moves only a few millimeters. A mobile bearing insert is able to accommodate the rollback of the lateral condyle but would not be necessary for the medial condyle.
Two primary difficulties exist with current joint replacement surgeries. These relate to the invasiveness of the procedure and achieving proper alignment of the bony structures and the prostheses thereupon.
Alignment
A difficulty with implanting both modular and non-modular knee implants having either separate femoral and/or tibial components has been achieving a correct relationship between the components. Surgical instruments available to date have not provided trouble free use in implanting multi-part implants wherein the femur and tibia are prepared for precise component-to-component orientation. While alignment guides aid in accurate orientation of the components relative to the axis of the long bones to achieve a restoration of a correct tibiofemoral alignment (usually 4-7 degrees valgus), they provide limited positioning or guidance relevant to correct component-to-component alignment and/or ligament tension to restore alignment.
It is preferable to orient implants normal to the resultant forces through the joint to subject bearing surfaces to compressive rather than shear forces. Moreover, the components of the implant are preferably oriented one to the other to minimize wear. Complications may result if the implant is not correctly oriented with respect to the supporting bone. If the implant is not placed normal to the mechanical axis, a shearing force results between the implant and bone that may lead to implant loosening.
In a properly aligned knee, the mechanical axis of the leg (a straight line drawn from the center of the hip joint to the center of the ankle) passes slightly medial to the center of the knee. This alignment is generally called the gross alignment of the leg. The alignment of the implants impacts the gross alignment of the leg. If the implants are malaligned, the resulting mechanical axis may be shifted medially or laterally, resulting in an imbalance in the loads carried by the medial or lateral condyles. This imbalance, if severe, may lead to early failure of the implant.
In addition, the orientation of the components to each other, for example the orientation of the femoral to the tibial component, with unicondylar and bicondylar implants has largely not been addressed. This may account for the high failure rates of early bicondylar designs and as well as for the higher failure rate of unicondylar implants relative to total knee implants as demonstrated in some clinical studies. When considering bicondylar and unicondylar designs, alignment of each part relative to the other parts is critical to avoid accelerated wear with a mal-articulation of the components.
Although various prosthetic devices have been successfully used with patients, the configuration and position of the articulating surfaces of the prosthesis, that is, for example, the condyles in a knee joint are predetermined based upon the prosthesis that is selected. While efforts are made to tailor the prosthesis to the needs of each patient by suitable prosthesis choice and size, this in fact is problematical inasmuch as the joint physiology of patients can vary substantially from one patient to another.
Invasiveness
In order to appropriately sculpt the articulating surface of a bone, it is often necessary to surgically expose the joint. In the case of the femur, the patellar tendon of the knee joint is surgically exposed and is moved to one side of the joint to enable a substantially full anterior access to the joint. Surgical exposure is necessary to accommodate the bulk and geometry of the components as well as the instruments for bone preparation. Such surgical exposure increases bleeding, pain, and muscle inhibition; all of which contribute to a longer hospitalization before the patient can be safely discharged to home or an intermediate care facility.
Desirably, in the case of knee replacement surgery, neither the collateral ligaments nor the cruciate ligaments are disturbed, although it is often necessary to remove or release cruciate ligaments in the event a substantial joint replacement is to be performed. Collateral ligaments can be partially taken down or released to provide appropriate tension adjustment to the patient""s knee in concert with joint replacement surgery. In most instances, such releases can be accomplished through smaller incisions than the standard midline or medial parapatellar incisions historically used for knee arthroplasty.
Arthroscopic surgery is available, and beneficial, for removing and repairing damaged intraarticular tissues. Although arthroscopic procedures are far less invasive and are often successful for minor surgical repairs, (as when an articular surface is to be smoothed, for example, or cartilage is to be repaired), such procedures generally are not appropriate for substantial joint replacement. They are generally inadequate for replacing joint surfaces with artificial implants.
Conventional surgical procedures including unicompartmental and total joint replacement historically require extensive surgical exposure and prolonged hospital stays and rehabilitation. More recently unicondylar procedures have been performed through smaller incisions that do not necessitate dislocation of the patella. The reduction in pain and more rapid recovery of knee function has reduced the length of hospital stay and the need for strong narcotic medications. It is desirable to realize such benefits for patients with bicompartmental and tricompartmental knee arthroplasty.
For patients who require articular surface replacement, including patients whose joints are not so damaged or diseased as to require whole joint replacement, it would be desirable to provide surgical methods and apparatuses that could be employed to gain surgical access to articulating joint surfaces, to appropriately sculpt the surfaces, to provide artificial, e.g., metal or plastic, articular bearing surfaces, and then to close the surgical site, all without substantial damage or trauma to associated ligaments and tendons. To reach this goal, of course, a procedure must be provided to enable articulating surfaces of the joints to be appropriately sculpted using minimally invasive apparatuses and procedures.
The present invention provides an implant system that restores individual patient joint kinematics and involves minimally invasive surgical procedures. The instruments and implants disclosed accomplish accurate bone and soft tissue preparation, implant orientation and implant fixation through limited surgical exposure. The implant system is comprised of implants and instrumentation that provide intraoperative surgical options for articular constraint and facilitate proper alignment and orientation of the knee to restore kinematics as defined by the individual patient anatomy. To do so, the implants provide a surgeon intraoperative options to reconstruct various degrees of joint stability via selection of fixed or mobile bearing components for each compartment of the knee (medial tibiofemoral joint, lateral tibiofemoral joint and patellofemoral joint). The range of implants may cover each compartment of the knee and may include combinations of fixed and mobile bearing configurations.
In traditional total knee replacements, the femoral component is generally a unitary piece and the tibial component is a unitary piece. In the current invention, the femoral side may be resurfaced by two or three components and the tibial side may be resurfaced by two components or a unitary piece. Optionally, the components of the femoral side may be comprised of a plurality of flexible segments.
Proper alignment and positioning of the implant components is facilitated by instrumentation that utilizes the soft tissue structures of the knee to guide bony resections for patient-specific alignment and orientation of the implants. The surgical instrumentation prepares the articular surfaces of a synovial joint from a single point of reference to allow the introduction of separate components for the medial and lateral tibiofemoral compartments, and the patellofemoral compartments with precise orientation. Thus, the instrumentation provides bony resections in accordance with such alignment and orientation requirements. The alignment positioning is important for proper restoration of anatomic alignment of the knee joint and for proper orientation of the components to one another.
With respect to forming or sculpting articular surfaces of a joint, the method of the current invention enables the articular bone surfaces to be sculpted according to the individual physiology of each patient to restore as much as possible the natural junction of the joint. In this method, a bone sculpting tool is attached to one of the bones of a joint, and the tool sculpts the articular surface of the other bone as the joint is articulated.
Thus, in one embodiment, the present invention provides a method of appropriately sculpting the articular surface of a first bone that normally articulates with a second bone. The method involves providing an apparatus comprising a bone sculpting tool attached to a bone mount, attaching the mount rigidly to the second bone with the tool in bone sculpting engagement with the articular surface of the first bone, and then sculpting the articular surface by articulating one of the bones with respect to the other.
In some situations, it may be desirable to distract the first bone from the second bone either preoperatively or during surgery. Thus, a distractor may be provided with the apparatus. A distraction force provided between the femur and the tibia during the sculpting procedure accounts for material that has worn away from the articular surfaces. Use of a distraction force generally re-establishes normal alignment of the joint. Additionally, a distractor may be used preoperatively to assess the range of motion of the joint and patient kinematics.
In another embodiment, the invention provides an apparatus for sculpting the articular surface of a first bone that normally articulates in a predetermined manner with the second bone. The apparatus comprises a bone sculpting tool, a mount attachable rigidly to the second bone, and an adjustable attachment attaching the sculpting tool to the mount and enabling the position and orientation of the tool to be adjusted into bone-sculpting proximity to the articular surface so that the articular surface is sculpted as the second bone is articulated with respect to the first bone. Alternately, a plurality of bone sculpting tools may be used where the tools are positioned either on individual mounts or on a single mount to support the plurality of tools.
The invention also provides implants for replacing the surfaces of the joint between the first bone and the second bone. The implants are specifically designed to fit through minimally invasive incisions and incorporates any and all combinations of fixed and mobile bearing inserts or parts. Since the surgical procedure preferably is performed through minimally invasive incisions the implants are designed to fit through such incisions and be either oriented or joined within the joint.
The implants include a second bone baseplate and a first bone implant. The second bone baseplate may be either one piece to cover most of the prepared surface of the second bone as relates to the joint, or separate baseplates as have been used with mobile and fixed bearing prosthetic components. In addition, the second bone baseplate may accommodate separate fixed and mobile bearing inserts. The first bone implant is comprised of a plurality of components to replace the bearing surface of the first bone. Optionally, a portion of the first bone implant may be configured of a plurality of flexible segments bonded in place. Such a configuration permits the articulation of the second bone to the first bone to mould the flexible segments in appropriate position.
Thus, in a further embodiment, the invention provides a method of appropriately replacing the articular surface of a first bone that normally articulates with a second bone. The method involves providing an apparatus comprising a bone sculpting tool attached to a bone mount, attaching the mount rigidly to the second bone with the tool in bone sculpting engagement with the articular surface of the first bone, and then sculpting the articular surface by articulating one of the bones with respect to the other. Further, resurfacing the articular surfaces with appropriate minimally invasive implants wherein the implants are joined within the confines of the joint cavity. In one embodiment, a plurality of flexible segments are provided to resurface a portion of the first bone. The flexible segments are set in an adhesive along the resected surface of the first bone.
Specifically, for example, the invention may be used for replacing the surfaces of a femur and a tibia. Thus, a femoral implant having a plurality of components and a tibial baseplate are provided. The tibial baseplate may have a fixed bearing attachment as well as a mobile bearing attachment.